Cook the macaroni according to the packet instructions in a large pan of salted boiling water, then rinse with cold water, drain and set aside.
Place the chopped sweet potato in a large pan of boiling water and cook until soft. Drain and set aside.
Pour the milk into a small pan along with the onion. Slowly bring to the boil, then remove from the heat. Remove the onion and discard.
To make the sauce, heat the olive oil in another pan over a medium heat, then add the flour, stirring continuously until it forms a paste, or roux. Gradually add the warm milk a little at a time whisking continuously until smooth. Bring the mixture to the boil then simmer for about 5 minutes, stirring constantly until the mixture is thick.
Stir in the mustard, dried rosemary and yeast flakes until well combined. Add sea salt and freshly ground black pepper to taste. Remove from the heat.
Set up the Easy Prep Pro with the large work bowl and the large chopping/mixing blade. Add the cooked sweet potato and the sauce to the work bowl. Process on ‘High’ until all the ingredients are combined and the sauce is creamy and smooth. Add a little extra milk if the sauce is too thick.
Place the macaroni in an oven proof bowl and pour over the sauce. Stir to combine if necessary.
Place in a hot oven for 20 minutes, until bubbling.
Scatter the top with fresh rosemary before serving.